VMS Help PSM Routines, USER-FORMAT-ROUTINE, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    When used, the func_arg_1 argument describes carriage-control
    information for the input data record, and the func_arg_2
    argument describes carriage-control information for the output
    data record.

    The input data record is passed to the format routine (input
    filter or output filter) for processing, and the output data
    record is returned by the format routine (input filter or output
    filter).

    One of the tasks performed by the main format routine (routine
    code PSM$K_MAIN_FORMAT) is that of embedding the carriage-control
    information (specified by func_arg_1) into the data record
    (specified by func_desc_1). Thus, the output data (specified
    by func_desc_2) contains embedded carriage control and is thus no
    longer in record format; it is, therefore, properly referred to
    as an output data stream rather than an output data record.

    Similarly, the output filter routine (routine code PSM$K_OUTPUT_
    FILTER), which executes after the main format routine, uses
    neither the func_arg_1 nor func_arg_2 argument; the data it
    receives (via func_desc_1) and the data it returns (via func_
    desc_2) are data streams, not data records.

    However, the input filter routine (routine code PSM$K_INPUT_
    FILTER), which executes before the main format routine, uses
    both func_arg_1 and func_arg_2. This is so because the main
    format routine has not yet executed, and so the carriage control
    information has not yet been embedded in the data record.